This invention relates to certain novel metal-modified phenolic resins and to methods for preparing such resins. More particularly, the invention relates to novel metal-modified phenolic resins which are useful in the color developer part of a carbonless copy paper system.
Carbonless copy paper is widely used today, particularly in various business forms where one or more copies are needed. Carbonless copy paper is preferred over copies made by conventional carbon paper for a variety of reasons, including ease of handling and various expenses associated with the use of carbon paper. Carbonless copy paper is also particularly adapted for use in various computer generated documents.
As the name implies, a carbonless copy paper system generally consists of two or more sheets of paper: an original or top sheet on which an image may by formed by a pen, a typewriter, a printer, etc. The second sheet is the copy, on which a copy of the original image is generated. Generally, the backside of the top sheet or the original is coated with a color former coating, while the top side of the copy sheet is coated with a color developer coating. Typically, the color former coating comprises microcapsules of a normally colorless chromogenic material, such as Crystal Violet Lactone, and a solvent. These microcapsules are ruptured by the pressure applied during the formation of the image on the original, such as by the action of a pen or a typewriter, resulting in the localized release of the solution of the chromogenic material. The color developer coating, on the top surface of the copy sheet and facing the color former coating, typically contains a material which will react with the chromogenic material and solvent released by the color former microcapsules during the writing process to produce a visible image on the copy sheet. Such a carbonless copy paper system is disclosed, for example, in U.S. Pat. No. 2,712,507. One embodiment of the microcapsules containing the solution of chromogenic material is disclosed in U.S. Pat. No. 2,800,457.
It is also known to provide both the color former and color developer in a single coating coated on the side of the copy paper facing the original sheet. Such a so-called self-contained paper is disclosed in U.S. Pat. No. 2,730,457. The present invention is applicable to both these carbonless copy paper systems, but for purpose of discussion herein reference will be made to the commercially important two-coating system.
As indicated above, the color developer coating contains a material which will react with the released solution of chromogenic material to provide a visible image. Early color developer materials were acid clay-like materials such as attapulgite and zeolite materials or kaolin. See, e.g., U.S. Pat. No. 2,712,507. Subsequently, it was found that an improved color developer material may be obtained from a mixture of a phenolic polymer, such as a phenol-aldehyde polymer, commonly known as a novolak, with or without a clay material such as kaolin. See, e.g., U.S. Pat. Nos. 3,455,721 and 3,466,184. More recently, it has been found that the preferred phenolic polymers for use as color developers are those made with a para-substituted phenol. See, e.g. U.S. Pat. No. 3,663,256. Novolaks are known to be acidic and it is believed that the acid radicals in the novolaks interact with the color-forming chromogenic material to result in a colored material. However, an image so obtained has been shown by experience to have certain fading problem.
With the need in mind of a color developer which has a high speed of formation of the image, high image intensity, and long life or permanence of the image, it was found that the incorporation of a metal salt into the color developer gave improved results. See, e.g. U.S. Pat. Nos. 3,516,845 and 3,723,156. While said U.S. Pat. No. 3,723,156 discloses a color developer containing a metal salt whose anion contain at least four carbon atoms, such as zinc naphthenate or zinc 2- ethylhexanoate, U.S. Pat. No. 4,025,490 claims a method for producing a metal-modified phenol-aldehyde novolak resin which included the use of a metal salt whose anion contains 1-3 carbon atoms. U.S. Pat. Nos. 4,165,102 and 4,165,103 disclose that zinc modified phenol-aldehyde novolak resins for use as color developing agents can be prepared by reacting the phenolic resin with zinc oxide or zinc carbonate and an ammonium carboxylate salt such as ammonium benzoate or ammonium formate. Another refinement in the art of the color developer is disclosed in U.S. Pat. No. 3,737,410, which shows the modification of a para-substituted novolak resin with zinc dibenzoate and a weak base such as ammonium bicarbonate. It is believed that the weak base, such as an ammonium compound, serves to solubilize the metal salt present. However, it has been found that the performance of such a color developer fluctuated, depending on the efficacy of the color forming system employed.
Recent work on improving the color developer includes the use of an aromatic carboxylic acid, such as salicylic acid, in the color developer composition. Salicylic acid, though much more expensive than phenol, is alleged to give a much higher quality image when used in a color developer. Thus, U.S. Pat. No. 3,772,052 discloses a color developer comprising a metal compound of a polymer, the polymer being a reaction product of an aromatic carboxylic acid with an aldehyde. However, aside from the fact that salicylic acid is substantially more expensive than phenol, the polymerization reaction between salicylic acid and formaldehyde is also a slow and difficult one, as compared to the reaction of phenol and formaldehyde. Thus, more stringent conditions, such as by the use of large amounts of strong acid catalyst, are required for the polymerization reaction of salicylic acid and formaldehyde. In addition, since salicylic acid is relatively less soluble in many organic solvents, its use per se may retard the imaging speed of a color developer or a carbonless copy paper system.
U.S. Pat. No. 3,874,895 discloses a color developer comprising a mixture of an acidic polymer such as a phenolic polymer and an organic carboxylic acid or a metal salt of such an acid. U.S. Pat. No. 3,924,027 discloses an acceptor composition for a pressure sensitive copy system, which is a particulate mixture of an organic acid substance or a polyvalent metal salt of the acid substance and a polymer. U.S. Pat. No. 4,134,847 discloses a color developer which is a mixture of at least one aromatic carboxylic acid, at least one polymer, and at least one polyvalent metal oxide or carbonate. In these four patents, salicylic acid, if present, may be used as a component to be mixed with a polymer (such as in U.S. Pat. Nos. 3,874,895, 3,924,027 and 4,134,847) or may be reacted with formaldehyde to form the polymer in the color developer composition (such as in U.S. Pat. No. 3,772,052 and 3,874,895). In addition, mixtures of phenolic polymers and salicylic acid/formaldehyde polymers may be used (such as in U.S. Pat. No. 3,874,895).
In U.S. Pat. Nos. 4,173,684 and 4,226,962 there are disclosed metal-modified novolak resins prepared by reacting a para-substituted phenol, salicylic acid and formaldehyde to form a melted novolak resin, which is then mixed with a zinc salt to form the metal-modified novolak resin.
The use of zinc salicylate with phenolic resins is disclosed in U.S. Pat. No. 3,624,038. There, zinc salicylate is used as a catalyst in the condensation of phenol and formaldehyde to form a composition suitable for use as an adhesive.
The manufacture of phenolic resins containing a benzyl ether structure is known in the art of foundry resin binders such as disclosed in U.S. Pat. Nos. 3,485,797 and 3,676,392. The resins are made with metal salt catalysts such as zinc naphthenate, and stannous octoate. These resins generally have a high ratio of formaldehyde to phenol, characteristics of resole resins, when used in their final form as foundry sand binders. The resins are generally unsubstituted in the para-position, although para-substitution has been disclosed.
Canadian Pat. No. 927,041, and U.S. Pat. No. 4,036,816 and 4,045,411, disclose phenolic resins having the benzyl ether structure made with zinc catalysts, such as zinc formate and zinc acetate. Unsubstituted phenol is preferred as a reactant and the products, to be used as adhesives, have a high ratio of formaldehyde to phenol in their final form.
Although the color developers of the prior art provide acceptable carbonless copies, there is a commercial need for improved and economical color developers.
Accordingly, it is an object of the present invention to provide an improved and economical color developer for a carbonless copy paper system having high image intensity and high speed of image formation.
It is another object of this invention to provide a color developer which will form an image that is resistant to fading.
It is a further object of the invention to provide a novel metal-modified phenolic resin, for use as a color developer, that is readily emulsifiable or dispersible.
These and other objects of the invention will be apparent from the following description.